1. Field of the Invention
The present invention relates to a photocurable composition, a process for producing photocurable resin, and a crosslinking product thereof. More specifically, the composition of the present invention is usable for coatings, printing inks or molded articles which are curable with UV light to day light without any addition of photoinitiators.
2. Description of Related Art
Monomers, oligomers and polymers containing acrylic acid ester groups, which are also termed acrylates, may be hardened by exposure to UV (ultraviolet) light. For this curing system, a photoinitiator is, as a rule, necessary, and forms free-radicals under photoirradiation and initiates free-radical polymerization of the acrylate groups, which then leads to a hardening of the product. Monomers, oligomers and polymers containing acrylate groups as well as photoinitiators are essential constituents of radiation-hardening systems that are used as coatings, printing inks, adhesives, or molding compositions.
However, the above-mentioned radiation-hardening systems containing photoinitiators are not entirely problem-free. For example, unreacted photoinitiators as well as their cleavage products remain in the coating and may cause various problems. For example, residues of photoinitiators and cleavage products can migrate through polymeric coatings and affect adjacent products. In the foodstuff packaging industry, this may lead to contamination of the foodstuffs themselves, which has to be avoided (U.S. Food and Drug Administration, “Recommendations for Chemistry Data for Indirect Food Additive Petitions”).
Furthermore, residues of photoinitiators such as, for example, benzophenone, or volatile cleavage products such as, for example, benzaldehyde, that are formed by free-radical cleavage of α-cleavable compounds may in addition lead to the formation of an odor that adversely affects the quality of the hardened products.
Furthermore, serious problems that can be caused by photoinitiators include yellowing, which may occur when using amine-containing co-initiators, as well as an insufficient through-hardening in the case of thick layers as a result of strong light absorption by the photoinitiators.
On account of the aforementioned disadvantages, the need has arisen in the technology to provide acrylate group-containing systems that can harden in the absence of added and known photoinitiators.
As a means for overcoming these problems, there have been proposed a photoinitiator-free oligomeric composition having acryloyl groups. For example, U.S. Pat. No. 6,025,410 discloses liquid oligomeric compositions produced by Michael addition reaction of acetoacetates with acrylates, in which the composition can be polymerized under UV light in the absence of photoinitiators.
The liquid oligomeric compositions, however, have a problem in that the cured products thereof result in soft and tacky films (as described in column 12, lines 39-40). Although these properties are desirable for many applications, for example for adhesives, they are undesirable for other applications, for example as coating lacquers as well as for molded articles. Moreover, the storage stability of a few months of the liquid oligomeric compositions is still unsatisfactory.